The present invention relates to a crystal puller for growing single crystal semiconductor material, and more particularly to a heat shield assembly for use in such crystal pullers.
Single crystal semiconductor material, which is the starting material for fabricating many electronic components, is commonly prepared using the Czochralski ("Cz") method. In this method, polycrystalline semiconductor source material such as polycrystalline silicon ("polysilicon") is melted in a crucible. Then a seed crystal is lowered into the molten material and slowly raised to grow a single crystal ingot. As the ingot is grown, an upper end cone is formed by decreasing the pull rate and/or the melt temperature, thereby enlarging the ingot diameter, until a target diameter is reached. Once the target diameter is reached, the cylindrical main body of the ingot is formed by controlling the pull rate and the melt temperature to compensate for the decreasing melt level. Near the end of the growth process but before the crucible becomes empty, the ingot diameter is reduced to form a lower end cone which is separated from the melt to produce a finished ingot of semiconductor material.
Although the conventional Cz method is satisfactory for growing single crystal semiconductor materials for use in a wide variety of applications, further improvement in the quality of semiconductor material is desirable. For instance, as semiconductor manufacturers reduce the width of integrated circuit lines formed on semiconductors, the presence of microscopic defects in the material becomes of greater concern. Defects in single crystal semiconductor materials form as the crystals solidify and cool in the crystal puller. Such defects arise, in part, because of the presence of an excess (i.e., a concentration above the solubility limit) of intrinsic point defects known as vacancies and interstitials.
One important measurement of the quality of wafers sliced from a single-crystal ingot is Gate Oxide Integrity ("GOI"). Vacancies, as their name suggests, are caused by the absence or "vacancy" of a silicon atom in the crystal lattice. When the crystal is pulled upward from the molten silicon in the crucible, it immediately begins to cool. As the temperature of the crystal ingot decreases, the solubility limit decreases.
Point defects that are present at high temperatures then precipitate out in the form of microscopic defects (voids), or they move to the lateral surface of the crystal. This generally happens as the crystal descends through the temperature range of 1150.degree. C. to 1500.degree. C.
Silicon wafers sliced from the ingot and manufactured according to conventional processes often include a silicon oxide layer formed on the surface of the wafer. Electronic circuit devices such as MOS devices are fabricated on this silicon oxide layer. Defects in the surface of the wafer, caused by the agglomerations present in the growing crystal, lead to poor growth of the oxide layer. The quality of the oxide layer, often referred to as the oxide film dielectric breakdown strength, may be quantitatively measured by fabricating MOS devices on the oxide layer and testing the devices. The Gate Oxide Integrity (GOI) of the crystal is the percentage of operational devices on the oxide layer of the wafers processed from the crystal.
It has been determined that the GOI of crystals grown by the Czochralski method can be improved by increasing the amount of time a growing ingot dwells in the temperature range above 1000.degree. C., and more particularly in the range of 1150.degree. C.-1050.degree. C. If the ingot cools too quickly through this temperature range, the vacancies will not have sufficient time to agglomerate together, resulting in a large number of small agglomerations within the ingot. This undesirably leads to a large number of small voids spread over the surfaces of the wafer, thereby negatively affecting GOI. Slowing down the cooling rate of the ingot so that its temperature dwells in the target temperature range for a longer period of time allows more vacancies to form larger agglomerations within the ingot. The result is a small number of large agglomerations, thereby improving GOI by decreasing the number of defects present in the surface of the wafer upon which the MOS devices are formed.
Another way to improve GOI is to control the number of vacancies grown into the ingot. It is understood that the type and initial concentration of vacancies and self-interstitials, which become fixed in the ingot as the ingot solidifies, are controlled by the ratio of the growth velocity (i.e., the pull rate) (v) to the local axial temperature gradient in the ingot at the time of solidification (G.sub.o). When the value of this ratio (v/G.sub.o) exceeds a critical value, the concentration of vacancies increases. Likewise, when the value of v/G.sub.o falls below the critical value, the concentration of self-interstitials increases.
One way to increase this ratio is to increase the pull rate (i.e., growth velocity, v) of the ingot. However, an increase in pull rate will cause distortion in the diameter of the ingot if the ingot is given sufficient time to cool and solidify. To this end, it is known to position a heat shield assembly within the crucible above the melt surface between the crucible side wall and the growing ingot for shielding the ingot from the heat of the crucible side wall. The conventional heat shield assembly typically includes an outer reflector and an inner reflector. A schematic cross section of the wall of a conventional heat shield is shown in FIG. 2. The outer reflector OR is secured to the inner reflector IR by suitable fasteners (not shown) spaced along annular upper and lower fastener locations so that the outer reflector directly contacts the inner reflector at these locations. The outer reflector OR is substantially shorter than the inner reflector IR so that an upper portion of the heat shield assembly comprises a single, uninsulated layer. The reflectors OR, IR are shaped to define an insulating chamber therebetween containing insulation IN for inhibiting heat transfer from the outer reflector to the inner reflector.
The insulation I is intended to insulate a midportion M of the inner reflector IR against heat transfer from the outer reflector OR so that heat from the crucible wall is not transferred to the inner reflector. Providing a cooler portion of the inner reflector IR permits quicker cooling of the ingot as the ingot is pulled upward into radial registration with that portion of the heat shield. However, because of the large surface area contact between the outer reflector OR and inner reflector IR at the upper and lower fastener locations, a substantial amount of heat from the outer reflector is undesirably conducted directly to the inner reflector such that the inner reflector is not cooled as well as desired. This substantially limits the pull rate of the growing ingot.
An additional measurement of the quality of the wafers sliced from a crystal ingot relates to Oxygen Induced Stacking Faults (OISF). OISF results from defects being grown into the ingot as the ingot solidifies at the melt surface. The defects are a result of a differential between the axial temperature gradients at the center of the ingot and the outer surface of the ingot. OISF is measured as a ring spaced some distance inward from the peripheral edge of a wafer. The concentration of stacking faults within a particular area of the wafer surface may also be measured. The gradient at the interface varies with r, which leads to varying concentrations of point defects. Thus, by choosing the appropriate pull rate concentrations can be achieved, resulting in better OISF performance.